1. Field of the Invention
The present invention relates to rotary actuators for a direct access storage device (DASD), and more particularly to improved driving methods and rotary actuators for a high track density storage device.
2. Description of the Prior Art
Disk drive units incorporating stacked, commonly rotated rigid magnetic disks are used for storage of data in magnetic form on the disk surfaces. Transducer heads driven in a path toward and away from the drive axis write data to the disks and read data from the disks. Data is recorded in concentric data information tracks arrayed on the surfaces of the disks.
Known rotary actuators for positioning the transducer heads relative to the disk surfaces typically include a pivotable support member and a comb assembly including several closely spaced rigid arms. Attached to each rigid arm is at least one transducer head. Typically one of the rigid arms includes an extension driven in a pivotal motion by a voice coil motor. A voice coil cooperating with permanent magnets and core assembly is operatively controlled for moving the transducer heads in synchronism in a radial direction in order to position the heads in registration with the tracks to be followed.
For example, U.S. Pat. No. 4,697,127 discloses a voice coil motor driven disk file actuator. The disclosed actuator uses a single voice coil motor (VCM) and an adaptive controller system for controlling the coil current applied to the actuator VCM to control the position of the transducer head of the file. The basic modes of operation are track following and seek. Track following holds the transducer head over a desired track and seek moves the head to a desired track. A settle mode exists during the transition from the seek to the track following mode. Position and coil current measurements are used to generate a position error signal. The position error and an estimated velocity signal are used to generate the magnitude of the coil current during seek operations. In the track following mode, an integrated position signal, position and estimated velocity signals are combined to produce a composite signal corresponding to the current magnitude to be applied to the actuator VCM.
While known rotary actuators are adequate for present track density requirements, higher track densities or tracks per inch (TPI) for writing data on the disk surfaces require more precise movement. Parasitic and nonlinear movements resulting from non-ideal pivot design limit seek and track following performance of known rotary actuators. An ideal pivot has zero-stiffness and zero bias in the desired plane of motion and infinite stiffness in all other coordinate planes. Zero clearance between a sliding metal-to-metal interface required for an ideal pivot is impossible because a thin viscous lubricant film is necessary for proper lubrication. In rotary actuators incorporating a non-ideal pivot design based upon preloaded ball bearings or flexure springs, parasitic motion is generated in the opposite direction of the desired motion as a result of finite stiffness or fluid film gap.